This invention relates to hydraulic pressure control devices which generate a control pressure. The invention is specifically concerned with a hydraulic pressure control orifice of the type in which a solenoid moves a plunger with respect to an orifice seat to define a controlled restriction.
Hydraulic pressure control valves commonly use a hydraulic half bridge to generate a control pressure. The half bridge consists of two flow restrictions placed in series between a high pressure source and a low pressure sink, generating an intermediate control pressure in between. Typically, a plunger over an orifice is used as the second restriction, to generate an equilibrium between the control pressure and an input force. The control pressure multiplied by the area of the plunger (or orifice) equals the force generated in opposition to the input force. Equilibrium is maintained by allowing the plunger to move with respect to the orifice. Thus, an increase in input force causes the orifice to close, increasing the restriction, and increasing the control pressure. A decrease in input force allows the control pressure force to overcome the input force, allowing the plunger to move away from the orifice, reducing the control pressure. Likewise, an increase in control pressure, caused by a change in the load, causes the plunger to move away from the orifice to reduce the control pressure back to the equilibrium pressure. Similarly, a decrease in control pressure allows the input force to overcome to the force generated by pressure, close the plunger toward the orifice, and increase the control pressure back toward equilibrium.
The pressure control plunger and orifice, as described, performs two functions: It generates a force proportional to the control pressure, and it allows flow to pass (to maintain that pressure in the half bridge). Ideally, these two functions do not interact. In reality, the flow passing the plunger generates pressures which act on several regions of the typical plunger. Some of the resulting forces are desirable, some not. The plunger and orifice together form an annular vena contracta. Fluid approaching the orifice at low axial velocity produces a stagnation pressure on the front of the plunger substantially equal to the control pressure. This is the desired effect. Near the annulus, the flow accelerates as it approaches, and decelerates as it departs behind the vena contracta. The deceleration is caused by an adverse pressure gradient. The adverse pressure gradient of the downstream or diffuser part of the vena contracta can generate a low pressure at the throat of the vena contracta. This low pressure, acting on adjacent surfaces, generates forces known as "Bernoulli forces". These forces act perpendicular to the surface of the plunger, generating radial force on the sides of the plunger, and an axial force on the front of the plunger. These forces tend to draw the plunger off center, and also draw it forward toward the oncoming flow. These forces are undesirable. In addition, the decelerating flow can produce a force on the back of the plunger in the desired direction. However, the adverse pressure gradient can cause this flow to detach from the back of the plunger at various velocities, forming unsteady rotational flows and pressures. Therefore, these forces are also undesirable.